Controlled release devices are used to control plants, and pests. Most controlled release devices are a combination of an active chemical, for example a pesticide, with a polymer, for example polyethylene. Other controlled release devices include a carrier, for example carbon black.
South African patent 86133 lodged 14 FEB. 1986, entitled DRIP IRRIGATION SYSTEMS describes a "master batch" used for making drip emitters that prevent root intrusion. The master batch is described as a blend of 58 parts by mass of polyethylene powder with 18 parts by mass carbon black and heated to 70.degree. C. Separately, 24 parts by mass of trifluralin are heated to melt the trifluralin and the melted trifluralin is slowly added to the polymer/carbon black mixture to make a friable mixture having from about 20-33 wt % trifluralin. The friable master batch is further combined with 90% by mass synthetic resin making a master batch combination achieving about 1-5 wt % trifluralin. The master batch combination is then injection molded. This master batch process is particularly aimed at high tolerance devices wherein a dilution of the trifluralin within the master batch is required (i.e., 30 wt % to 5 wt % trifluralin).
However, it has been found that this master batch combination has problems during injection molding. Specifically, as the master batch combination passes through the injection molder, the temperature of the master batch increases thereby increasing the vapor pressure of the trifluralin so that vaporized trifluralin issues forth from the master batch creating bubbles and voids greater than about 10% by volume in a size range from 0.05 mm to 0.5 mm in the injection molded product. The problem of increased vapor pressure of the trifluralin was partially overcome by increasing mixing time of the master batch combination prior to injection molding. However, the increased mixing time did not fully eliminate the bubbles and voids. In addition to production problems, the bubbles and voids caused a reduction in longevity of the product. When voids and bubbles were present, the active chemical was-released faster than desired, and the release rate was less predictable.
Thus, there is a need for a method of making a controlled release device that is moldable without flaws, bubbles and voids, and specifically there is a need for a method of making a controlled release device that is injection moldable.